The present invention relates to a progressive power lens having a mark printed thereon that indicates the location for measuring the dioptric power thereof.
FIG. 1A schematically shows typical structure of a conventional progressive power lens 100. The progressive power lens 100 typically has a distance portion 111 having a dioptric power for distance vision at an upper area thereof, a near portion 112 having a dioptric power for near vision at a lower area thereof, and an intermediate portion 113 between the distance portion 111 and the near portion 112. The dioptric power in the intermediate portion 113 progressively varies from the upper portion to the lower portion. Such variation in dioptric power is given by an asymmetrical shape formed on the front or back surface of the progressive power lens 100. The surface provided with the asymmetrical shape above is generally referred to as a progressive side surface.
An imaginary line called main meridian 116 extends through the distance portion 111, the intermediate portion 113 and the near portion 112 substantially along a vertical center line of the progressive power lens 100. The progressive power lens 100 is designed such that the aberration thereof is well corrected along the main meridian 116 since the line of sight of a lens wearer frequently moves therealong.
Generally, the main meridian 116 in the near portion 112 is displaced inwardly, or toward a nasal side, from that in the distance portion 111 since the eyes of the lens wearer turns inwards when observing objects at near distance. The horizontal distance H of the displacement mentioned above is known as an amount of “inset”.
FIG. 1B is a front view of the conventional progressive power lens 100. The progressive power lens 100 is generally provided with a printing pattern 120 including a plurality of marks by means of pad printing, for example. The printing pattern 120 includes, for example, a left/right identification mark 121 which indicates for which eye (left or right) the lens is designed, a cross 122 indicating the fitting point position, alignment reference marks 123 indicating the horizontal direction of the lens 100, and distance and near reference area marks 124 and 125. The distance and near reference area marks 124 and 125 are circles for indicating areas, i.e. distance reference area and near reference area, where the dioptric power of the distance portion 111 and near portion 112 should be measured.
The printing pattern 120 is designed such that the distance and near reference area marks 124 and 125 are printed in the distance and near portions 111 and 112, respectively, with their centers positioned on the main meridian 116. However, the position of the main meridian 116 within the near portion 112, or the amount of inset, varies in accordance with the dioptric power at the near portion 112. For example, if the dioptric power is negative, then the amount of inset is relatively small, while the inset amount becomes relatively large when the dioptric power is positive. Accordingly, the printing pattern 120 has to be redesigned for each progressive power lens having a different dioptric power which in turn causes increase in cost of the progressive power lens.